Overview
An analogy can explain a problem which the invention solves. If car radio is playing softly, a person can hear it while driving slowly. However, as speed increases, the radio volume must be turned up, because the background noise of wind, trucks, etc., will drown the radio's sound.
Similarly, a reluctance sensor 3 (analogous to the radio) in FIG. 2 produces an electrical signal when a slot 6 in wheel 9 passes it. (The passing slot causes the signal because the wheel and the sensor are inductively coupled together, like a transmitting and a receiving antenna. The passing slot disrupts the coupling, or mutual inductance, and produces the signal.)
At low speeds, the reluctance signal is small, as indicated in FIG. 4A. (The signal depends on the rate of change of inductance, dL/dt. For a slowly passing slot, dL/dt is small.) At low speeds, background noise is also small. (Background noise is caused by factors such as vibration. Vibration rocks the sensor back and forth from the wheel, and introduces its own change in coupling. At low speeds, the rocking is small.)
At higher speeds, the reluctance signal is higher, as in FIG. 4C (because the slot passes by the sensor faster), and background noise is also higher (because the rocking is greater).
This higher background noise at high speed is the problem: If a detector is sensitive enough to respond to small reluctance signals at low speeds, then, at high speeds, the detector will respond to both the background noise as well as the reluctance signals.
That is, the detector cannot "hear" the reluctance signal behind the background noise, just as the radio listener cannot hear the soft radio behind the wind noise.